With reference to FIGS. 5 to 11, description will be made on examples of a conventional acoustic-electric converter (hereinafter called a light sound converter) for converting a vibrational displacement of a diaphragm caused by sounds into an electric signal by utilizing a change in light caused by the vibrational displacement. A diaphragm 7 of a light sound converter shown in FIG. 5 has a flat film made of an aluminum foil or the like having a thickness of, for example, about 2 μm, without performing molding, working and the like.
Light emitted from a light emitting element 3 is reflected at the surface of a planar diaphragm 7 and received by a light receiving element 4. In order to prevent reflected light from being disturbed, it is necessary to eliminate crinkling, sagging and the like and maintain flatness of the diaphragm 7 made of a very thin film. To this end, the diaphragm 7 of a planar film is formed by applying a predetermined tension to the diaphragm 7 to eliminate crinkling, sagging and the like.
While the thin film of the diaphragm is stretched, a ring 2 is adhered to an outer peripheral area of the diaphragm by means of adhesive or the like. Therefore, the thin film constituting the diaphragm is maintained flat by a predetermined tension and the outer peripheral area of the diaphragm is fixed to the ring 2. The ring 2 is fixed to a frame 9 which supports a light emitting element 3 and a light receiving element 4. A laser beam radiated from the light emitting element 3 and reflected at the diaphragm 7 is received by the light receiving element 4 to detect a displacement of the diaphragm 7 from an output of the light receiving element 4.
Since tension is applied to this conventional planar diaphragm 7, there arises a problem of a small compliance of the diaphragm 7. The diaphragm 7 is therefore harder than usual. A vibration amplitude of the diaphragm 7 therefore becomes smaller than usual, i.e., the vibration amplitude from which a wide range of frequencies is to be detected becomes smaller than usual.
To solve this problem, another method of forming a diaphragm has been proposed. According this method, a metal thin film is vapor-deposited to a thickness of, for example, about 1 μm on a surface of a wafer or the like. A ring is adhered to the metal thin film or foil formed on the wafer surface, and the ring together with the adhered metal thin film is removed from the wafer to obtain the diaphragm. Even with this method, it has been found that a weak tension is applied to the diaphragm because adhesive on the adhesive surfaces, particularly adhesive reaching to the inner peripheral area of the ring, shrinks when it is hardened.
Generally, the frequency characteristics of a planar diaphragm are not flat and the diaphragm resonates at its specific frequency to generate a peak. It is well known that the vibration amplitude of the diaphragm becomes small at the frequency other than the resonance frequency. It is difficult to obtain a vibration amplitude sufficient for detecting a wide range of frequencies, resulting in a narrow reproduction band of the light sound converter for detecting a vibration of the diaphragm by utilizing light.
To overcome this disadvantage, the present applicant has already proposed a method illustrated in FIG. 6. According to this proposal, a reflective part 8a of a diaphragm 8 has a dome shape, a suspension part 8b is formed continuously with an outer peripheral area of the reflective part 8a, and an adhesion margin part 8f is integrally formed with an outer area of the suspension part 8b. This adhesion margin part 8f is adhered to a frame 9. A laser beam radiated from a light emitting element 3 fixed to the frame 9 shown in FIG. 6 and reflected at the reflective part 8a is received by a light receiving element 4 fixed to the frame 9 to detect a displacement of the reflective part 8a from an output of the light receiving element 4.
This proposal also refers to a diaphragm 10 which has a large area suspension part 10c as shown in FIGS. 7 to 9 in order to obtain a higher vibration amplitude. A reflective part 10a of a dome shape is disposed in the central area of the diaphragm 10. A laser beam radiated from a light emitting element 3 fixed to a frame 9 and reflected at the reflective part 10a is received by a light receiving element 4 fixed to the frame 9 to detect a displacement of the reflective part 10a from an output of the light receiving element 4.
A rising part 10b rising obliquely at 45° is continuously formed with an outer peripheral area of the dome-shaped reflective part 10a. The suspension part 10c is formed integrally with the outer peripheral area of the rising part 10b, and constituted of five concentrical corrugations extending radially and each defined by a curve having a predetermined radius.
A vertical rising part 10e is formed continuously with the outer peripheral area of the suspension part 10c, and an adhesion margin part 10f is integrally formed with and horizontally protruded from the outer periphery of the rising part 10e. This adhesion margin part 10f is adhered to a ring 2. Although not shown, the ring 2 is formed integrally with the frame 9.
As well known, recent requirements of making compact a light sound converter are very severe. In order to meet this compactness requirements, the diameter of a diaphragm with a dome-shaped reflective part, a corrugation type suspension part and the like is required to be short. A compliance of the diaphragm becomes inevitably small.
The experiments made by the present inventors have verified that as the same sound pressure is applied to a diaphragm, the vibration amplitude becomes small in inverse proportion with the square of the area of the diaphragm. As the vibration amplitude of the diaphragm becomes low, it is obviously difficult, for the light sound converter for detecting a vibration displacement of the diaphragm by utilizing light, to correctly detect the vibration amplitude.
As a method capable of overcoming this disadvantage, it is easily conceivable to thin the base material of a diaphragm having a dome shape integrally formed with the reflective part 10a, suspension part 10c and the like. For example, a metal foil or resin film having a thickness of 4 μm is replaced by a metal foil or resin film having a thickness or 3 μm or thinner.
With this approach, however, although it is effective to improve the compliance of the suspension part 10c, the dome shape reflective part 10a becomes more likely to resonate as the base material is made thin. Namely, since the diaphragm is thin, a number of split resonances occur in the reproduction vibration frequency band. This approach is therefore associated with the disadvantage that there is a large strain in the reproduction band and the sound quality is degraded very much.
It is well known that the thinner a metal foil, a film or the like of the base material of a diaphragm is made, the vibration amplitude of the diaphragm becomes higher. The experiments made by the present inventors have confirmed that a thinning rate of a diaphragm is smaller than a raising rate of a vibration amplitude of the diaphragm. It is also well known that as a film is made thinner, a mechanical strength weakens which is necessary for maintaining the original structure of the film. Very delicate works of handling such a film during manufacture are therefore required and the productivity is degraded.
In order to obtain a higher vibration amplitude of a diaphragm having a predetermined area and made of a base material providing a strength eliminating resonance during vibration, it is desired to increase the compliance of only the suspension part so as to allow the reflective part to easily vibrate, i.e., it is desired to provide the suspension part with a high elasticity.
A general approach to improving the compliance of a suspension part of a diaphragm made of a film already exists. According to this approach, the area of the suspension part is reduced to provide the suspension part with a high elasticity.
For example, the suspension part is made narrow like a cantilever which supports the reflective part of the diaphragm. The present inventors have formed a cantilever suspension part 11c on a diaphragm 11 which is partially removed as shown in FIGS. 10 and 11, i.e., the diaphragm with its hatched portions being removed.
Description will be made on the shape of the diaphragm 11 before removing the hatched portions. A rising part 11b rising obliquely at 45° is continuously formed with an outer peripheral area of a dome-shaped reflective part 11a. The suspension part 11c is formed integrally with the outer peripheral area of the rising part 11b, and constituted of five concentrical corrugations extending radially and each defined by a curve having a predetermined radius.
A vertical rising part 11e is formed continuously with the outer peripheral area of the suspension part 11c, and an adhesion margin part 11f is integrally formed with and horizontally protruded from the outer periphery of the rising part 11e. This adhesion margin part 10f is adhered to a ring 2. The cantilever suspension part 11c formed in this manner had most of the area around the dome shape reflective part 11a of the diaphragm 11 being removed as shown in FIG. 10. The cantilever suspension part 11c also had a large width.
The reason for this is ascribed to the shapes of trimming molds determined by taking into consideration the productivity of partially removing the diaphragm through press trimming. More specifically, in order to retain the strength of trimming molds, a punch and a die, it is required that the punch and die have allowable minimum widths. This restriction of the minimum widths results in the limited shape of the cantilever suspension part 11c. The shape of the cantilever suspension part 11c is therefore limited by a press trimming method currently used in most common.
Measurements were conducted for a vibration amplitude of the diaphragm 11 having the cantilever suspension part 11c and for a vibration amplitude of a diaphragm before partial removal of the diaphragm by trimming molds. For the measurements, the adhesion margin part of the diaphragm was clamped with a diaphragm mounting jig having a predetermined ring shape. The diaphragm mounted on the jig was placed in front of a speaker to vibrate the diaphragm. A sound pressure of 94 dB at a frequency of 1 KHz was applied to the diaphragm from the speaker. The vibration amplitude of the diaphragm vibrated in this state was measured by a laser doppler method.
The measurement results indicated that the diaphragm with the cantilever suspension part 11c had a vibration amplitude slightly higher than that of the diaphragm without the cantilever suspension part. In terms of numerical values, the diaphragm without the cantilever suspension part had an amplitude of 0.076 μm to 0.1μm, whereas the diaphragm with the cantilever suspension part 11c had an amplitude of 0.08 μm to 0.12 μm. This difference is small, verifying that the effects of the cantilever part are less.